Antibiotic resistance in Neisseria gonorrhoeae remains a very important problem. Penicillin and tetracycline, which were once the antibiotics of choice for treatment of gonococcal infections, are no longer used due to the preponderance of strains resistant to these agents. Resistance to currently recommended antibiotics is also increasing. My laboratory is interested in the mechanisms of chromosomally-mediated antibiotic resistance in the gonococcus, especially those that promote high- level resistance and subsequent treatment failure. Intermediate- level chromosomally-mediated resistance to penicillin and tetracycline is due to three resistance loci. These include the penA gene encoding altered forms of penicillin-binding protein 2 (PBP 2), the mtr loci conferring resistance to hydrophobic agents, and the penB gene, which decreases outer membrane permeability. The genes involved in mediating high-level penicillin resistance, however, have been difficult to identify. Our work during the last funding period has identified two resistance genes, ponA and penC, which together mediate high- level penicillin resistance, and a third gene, tetGC, which confers high-level tetracycline resistance. This proposal outlines experiments to clone and characterize the penC and tetGC genes and to elucidate the mechanisms by which they increase resistance. In addition, we propose experiments that follow up on our structure/function studies of the penB gene product, porin IB, to understand how mutations in this protein increase both penicillin and tetracycline resistance. We also propose studies to complete our work on the crystal structure of penicillin- binding protein 2 (PBP 2), an essential penicillin target, and several mutant forms that display a lower affinity for beta- lactam antibiotics. In addition, we will engage in new structural studies of wild-type and mutant forms of porin IB to explicate in molecular detail how mutations in this protein decrease antibiotic permeability. The combination of genetic, biochemical, biophysical, and structural approaches outlined in this proposal will provide important insight into the mechanisms by which this important human pathogen becomes resistant to antibiotics.